The present invention relates to a safety device for an extracorporeal blood treatment machine, in particular a hemodialysis machine, a hemofiltration machine or a hemodiafiltration machine and a method of increasing the safety of an extracorporeal blood treatment machine.
Various methods of surgical blood purification or blood treatment are used to remove substances that are usually eliminated with the urine and to withdraw fluid from patients with chronic renal failure. Diffuse mass transport is predominant in hemodialysis (HD), and convective mass transport through the membrane takes place during hemofiltration (HF). Hemodiafiltration (HDF) is a combination of both methods.
Removing excess body fluid by ultrafiltration is a part of dialysis therapy. This process usually leads to a reduction in the patient""s blood volume. It the blood volume is reduced too much, symptomatic hypotension occurs, a frequent side effect of dialysis therapy. The tolerance of patients to volume reduction differs greatly from individual to individual: a drop in blood pressure occurs with a higher incidence in critical patient populations, such as diabetics and arteriosclerosis patients.
With current standard therapy, either a fixed ultrafiltration rate UFR or a fixed rate over time (UF profile) is preset. The reduction in blood volume is not measured, and only if problems occur is the ultrafiltration rate reduced manually. Optionally blood volume may be replaced by infusions.
It has been proposed that the patient""s blood volume be monitored during the extracorporeal blood treatment and that the ultrafiltration rate be adjusted in such a way as to prevent a reduction in blood volume, which would not be tolerated in terms of the patient""s circulatory stability. A technical prerequisite for this method is a sufficiently exact sensor technology for determination of the blood volume.
The volume to undergo ultrafiltration during the treatment, i.e., the total ultrafiltrate volume UFVtot, and the ultrafiltration rate UFR, affect the fluid compartment of the body as well as circulatory regulation, as mentioned above. Therefore, the process of ultrafiltration and thus also blood volume regulation are critical with regard to safety. In uncontrolled withdrawal of fluid at the rate of several L/h, a critical blood volume in terms of circulatory regulation is reached after only a few minutes, causing a severe drop in blood pressure requiring intensive medical measures.
Because of the possibility of a malfunction of the sensors and of the blood volume regulation control algorithm, it to date could not be guaranteed that there would not be any greater risk for the patient with these blood monitoring methods than with standard therapies where the physician assumes responsibility for the selected ultrafiltration rate or the ultrafiltration profile. Therefore, the equipment available commercially has heretofore avoided regulating the ultrafiltration rate as a function of the patient""s blood volume.
An object of the present invention is to create a safety device for an extracorporeal blood treatment machine which enables regulation of the ultrafiltration rate as a function of the blood volume without increasing the risk to the patient.
The present invention provides a safety device for an extracorporeal blood treatment machine, the treatment machine having a blood inlet line leading from the patient to the inlet of a first chamber of an exchange unit divided by a semipermeable membrane into a first and second chamber, and a blood outlet line leading from an outlet of the first chamber to the patient. An ultrafiltration device for withdrawing a predetermined total ultrafiltrate volume from the second chamber of the exchange unit at an ultrafiltration rate during a predetermined treatment time is provided, as is a measurement unit for determining the blood volume or a measurement quantity of the patient correlating with the blood volume. A control unit determines and controls an ultrafiltration rate as a function of the blood volume. The present invention is characterized in that the safety device has a computer unit for determining an upper limit for the ultrafiltration rate from the predetermined total ultrafiltrate volume and the predetermined treatment time and a monitoring unit for limiting the ultrafiltration rate controlled by the control unit to the upper limit when the ultrafiltration rate predetermined by the computer unit reaches the upper limit.
In addition, another object of the present invention is to indicate a method of increasing the safety of an extracorporeal blood treatment machine with a blood volume control.
The present invention also provides a method of increasing the safety of an extracorporeal blood treatment machine having a blood inlet line leading from the patient to the inlet of a first chamber of an exchange unit divided by a semipermeable membrane into a first and second chamber, a blood outlet line leading from an outlet of the first chamber to the patient, an ultrafiltration device for withdrawing a predetermined total ultrafiltrate volume from the second chamber of the exchange unit at an ultrafiltration rate during a predetermined treatment time, a measurement unit for determining the blood volume or a measurement quantity of the patient which correlates with the blood volume, and a control unit for controlling an ultrafiltration rate as a function of the blood volume. The method is characterized by the following process steps: determining an upper limit for the ultrafiltration rate from the predetermined total ultrafiltrate volume and the predetermined treatment time and limiting the ultrafiltration rate which is controlled by the control unit to the upper limit when the ultrafiltration rate predetermined by the computer unit reaches the upper limit.
The patient""s safety is increased by the fact that the ultrafiltration rate controlled as a function of the blood volume by the control unit of the extracorporeal blood treatment machine is limited to an upper limit UFRlim which is determined by the preset total ultrafiltrate volume UFVtot and the preset treatment time UFT. Basing the upper limit value on the ultrafiltration rate and the treatment time creates high safety for the patient without too greatly limiting the range within which the ultrafiltration rate can be varied.
In a preferred embodiment, the average ultrafiltration rate UFRM, which is necessary to withdraw the predetermined total ultrafiltrate volume UFVtot within the treatment time UFT, is determined by forming the quotient of the predetermined total ultrafiltrate volume UFVtot and the predetermined treatment time UFT. The upper limit value UFRlim for the ultrafiltration rate UFR is then determined by multiplying the average ultrafiltration rate UFRM by a factor xcex1. The factor xcex1 is preferably greater than 1.5 and smaller than 2.3.
Another preferred embodiment provides for the range of allowable ultrafiltration rates to be reduced progressively toward the end of the treatment. A reduction in the upper limit value for the ultrafiltration rate as the treatment time increases is advantageous in particular when using control algorithms which tend to select greater ultrafiltration rates at the beginning of the treatment than at the end of the treatment as a function of the blood volume, so that in accordance with experience the ultrafiltration tolerance is greater at the beginning of treatment than at the end of treatment. The reduction in the upper limit for the ultrafiltration rate may be an explicit function of the treatment time elapsed or the quantity of fluid already subjected to ultrafiltration.
In a preferred embodiment, the upper limit for the ultrafiltration rate drops during the treatment from a value determined by multiplying the average ultrafiltration rate by a factor xcex1 to the average ultrafiltration rate, with the factor xcex1 preferably being between 1.5 and 2.3.
In another preferred embodiment, the upper limit for the ultrafiltration rate UFRlim is based on the fact that greater ultrafiltration rates should not be allowed when only a low residual volume is to be removed. Therefore, the residual ultrafiltration rate UFRR(t) at which the ultrafiltrate volume can be withdrawn at a given point in the time frame still available in the treatment time UFT is determined.
By multiplying the residual ultrafiltration rate by a factor xcex1, which is advantageously between 1.5 and 2.3, the upper limit UFRlim (t) at this time is then determined. Basing the upper limit for the ultrafiltration rate on the residual ultrafiltration rate leads to higher rates being allowed only when they are necessary for withdrawing the residual volume. In comparison with a linear drop in the upper limit for the ultrafiltration rate, this limit has an effect in particular when a large portion of the volume has already been removed, i.e., in the critical phase of approaching the dry weight. An additional reduction in the maximum allowed rate is achieved here.
Monitoring the ultrafiltration rate controlled by the control unit of the blood treatment machine not only has the goal of protecting the patient from unphysiologically high ultrafiltration rates but also ensures that the patient will reach the desired dry weight. If the ultrafiltration rate is limited to an upper limit, there is the possibility that the volume to be filtered cannot be withdrawn within the predetermined treatment period.
Therefore, in another preferred embodiment, a warning device is provided, having a comparator device for comparing the residual ultrafiltration rate UFRR(t) with the upper limit UFRlim and/or with the average ultrafiltration rate UFRM. After exceeding the average ultrafiltration rate UFRM and/or the upper limit UFRlim, the warning device delivers a warning signal by which the nursing staff is alerted to the fact that there are problems in reaching the desired dry weight of the patient. The user then has an opportunity to extend the treatment time and/or reduce the target volume or tolerate smaller blood volumes than originally planned or to complete the treatment with the remaining average ultrafiltration rate without blood volume control.
Since the ultrafiltration rate is to be limited only on exceeding a lower limit, the monitoring unit of the safety device preferably has a comparator device for comparing the ultrafiltration rate controlled by the control unit with the lower limit value. The monitoring device restricts the ultrafiltration rate to the upper limit UFRlim only when the ultrafiltration rate is greater than the lower limit.